Also called an “alt attribute,” “alt text,” or “alt description,” an alt tag is the text used within an HTML code to provide a description of an image. Search engines cannot interpret images, so alt tags help improve SEO by helping search engines identify visual content that may be relative to a search term.
How Long Should an Alt Tag Be?
As is the case with most SEO-related efforts, your goal in creating an alt tag should be using just enough text to be specific, but not so much that the search engine crawlers penalize your site for keyword stuffing.
For example, this is too little: “reading”
This is too much: “reading a book read reading children reading children’s books picture book education read to your kids”
This is about right: “mother and child reading a book”